In general, high-purity polysilicon is widely used as a chemical or industrial source material in semiconductor devices, solar cells, etc., requiring semiconductor properties or high purity. Also, it is used in functional precision devices and small-sized, highly-integrated precision systems.
High-purity polysilicon is prepared by repeated silicon deposition on the surface of silicon particles based on thermal decomposition and/or hydrogen reduction of a highly purified silicon-containing reaction gas.
In commercial-scale production of polysilicon, a bell-jar type reactor has been mainly used thus far. Polysilicon products produced using the bell-jar type reactor are rod-shaped and have a diameter of about 50-300 mm. Preparing polysilicon using a bell-jar type reactor based on electrical resistance heating cannot be executed in a continuous manner because there is a limit in increaseing the rod diameter according to silicon deposition. In addition, the deposition efficiency is poor because the surface area required for silicon deposition is restricted and also excessive thermal loss results in high power consumption per unit volume of the product.
To solve these problems, a silicon deposition process using a fluidized bed reactor to produce polysilicon in the form of granules, i.e., particles having a size of about 0.5-3 mm, has been developed recently. According to this method, a gas supplied from the bottom to the top of the reactor forms a fluidized bed in which silicon particles are fluidized. The silicon particles become enlarged as silicon elements deposit out of the silicon-containing reaction gas which is introduced into the hot fluidized bed.
As in the bell-jar type reactor, such Si—H—Cl-based silane compound as monosilane (SiH4), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4), and a mixture thereof is used in the fluidized bed reactor as the silicon-containing reaction gas. Usually, the reaction gas further comprises at least one gas component selected from hydrogen, nitrogen, argon, helium, etc.
To achieve silicon deposition for production of polysilicon, the reaction temperature, or the temperature of the silicon particles, should be maintained at about 600-850° C. for monosilane, and at about 900-1,100° C. for trichlorosilane, which is the most widely used in commercial-scale production.
During the silicon deposition process, a variety of elementary reactions occur in the process of thermal decomposition and/or hydrogen reduction of the silicon-containing reaction gas. Further, the silicon elements grow into particles in different ways, depending on a composition of the reaction gas. Irrespective of the elementary reactions comprised therein and the ways of particle growth, the fluidized bed reactor yields a granular polysilicon product.
As silicon deposition and/or binding of the fine silicon particles continue, small silicon seed crystals grow in size and gradually sink toward the bottom of the fluidized bed. The seed crystals may be directly generated inside the fluidized bed reactor or may be supplied continuously, periodically or intermittently into the fluidized bed reactor after being prepared outside the fluidized bed. The polysilicon product, or the silicon particles, which are enlarged due to silicon deposition, may be withdrawn continuously, periodically or intermittently from the bottom of the reactor.
The fluidized bed reactor is advantageous over the bell-jar type reactor in production yield, because of the large surface area at which silicon deposition can occur. Further, the granular polysilicon product may be readily handled in the silicon application processes, including single crystal growth, preparation of crystal block or film, surface treatment and modification, preparation of chemical materials for reaction or separation, shaping or pulverizing of silicon particles, etc., differently from the rod-shaped product. Besides, the granular polysilicon products allow such processes to be operated in a continuous or semi-continuous manner.
One of the most difficult steps in the continuous or semi-continuous production of granular polysilicon using the fluidized bed reactor is to heat the silicon particles in order to maintain the temperature required for the deposition. The following problems are involved in heating the silicon particles in order to maintain the temperature required for the deposition reaction, while minimizing impurity contamination of the silicon particles within the fluidized bed reactor. The reaction gas supplied to the fluidized bed reactor can lead to the silicon deposition at a temperature of about 300° C. or higher. But, since silicon deposition occurs on the wall of the reaction-gas heating means and the silicon deposit becomes accumulated as operation proceeds, the reaction gas cannot be sufficiently preheated before being supplied into the fluidized bed reactor. Further, since silicon deposition also occurs on the surface of the components of the reactor, which are constantly exposed to the hot reaction gas, the silicon deposit is naturally accumulated thereon. It is therefore difficult to sufficiently heat the silicon particles by the conventional method of heating the walls of the reactor, and also it is impossible to operate the reactor stably for a long period of time. In addition, few methods are available that enable the effective heating of the silicon particles while minimizing impurity contamination.
A variety of technical solutions have been proposed to solve these problems. Mostly, they are based on partitioning the inner space of the fluidized bed reactor into a reaction zone where the deposition occurs on the surface of the silicon particles and a heating zone for heating the silicon particles and indirectly heating the reaction zone through the heating zone.
In one of the methods of partitioning the inner space of the fluidized bed reactor into a heating zone and a reaction zone, a tube-shaped partitioning means is installed inside the layer of silicon particles, so that the outside space surrounding the partitioning means is heated by an external heater and the space inside the partitioning means becomes the reaction zone where the silicon deposition occurs. According to this method, as described in Japanese Patent No. 1984-045917 and U.S. Pat. Nos. 4,416,914 (1983), 4,992,245 (1991) and 5,165,908 (1992), etc., a continuously circulating fluidized bed is formed as the silicon particles move downward in the heating zone and they move upward in the reaction zone carried by the reaction gas. But, this method has the following problems. Because the partitioning means, which partition the reaction zone and the heating zone concentrically, has a diameter smaller than the outer diameter of the heating zone, silicon deposition and accumulation occur severely on the inner surface of the partitioning means which is exposed to the reaction zone, making it difficult to operate the reactor for a long period of time. Also, since the circulation of the silicon particles along the circumferential direction is non-uniform, the method is not suitable for a large-scale production.
As another method of partitioning the inner space of the fluidized bed reactor into a heating zone and a reaction zone, it is possible to locate the reaction gas outlet of the reaction gas supplying means in the bed of silicon particles so that the upper and lower spaces can be defined as the reaction zone and the heating zone, respectively, with the height of the reaction gas outlet being the reference for the partition. The silicon particles in the heating zone are heated to maintain the reaction temperature of the reaction zone. According to this method, a fluidizing gas which does not cause silicon deposition, such as hydrogen, is supplied from the bottom to the top of the reactor, so that all or a part of the silicon particles in the heating zone are fluidized. Further, the silicon particles in the reaction zone are fluidized by a reaction gas. As the silicon particles are intermixed at the interface of the two zones, heat is continuously transferred from the heating zone to the reaction zone. In relation to this, U.S. Pat. Nos. 5,374,413 (1994), 5,382,412 (1995), 6,007,869 (1999), 6,541,377 (2003) and 7,029,632 (2006) and Japanese Patent No. 2001-146412 disclose a method of dividing the space of bed of silicon particles into a reaction zone and a heating zone and heating the heating zone by the conventional method using an electrical resistance heater, microwave heating means, etc. to maintain the reaction temperature inside the reaction zone. However, considering the productivity problem of the fluidized bed reactor, or the fact that it is difficult to maintain the bed of silicon particles at a predetermined reaction temperature for large-sized reactors, a method capable of heating the heating zone more efficiently is required for the large-scale production of polysilicon using a fluidized bed reactor. Further, because much energy is used to heat the fast flowing fluidizing gas in the heating zone requires, the heating of silicon particles becomes inefficient.
In relation to this, U.S. Pat. No. 6,827,786 (2004) proposes a fluidized bed reaction system in which the upper and lower spaces in the bed of silicon particles are divided into a reaction zone and a heating zone and a tube heated by a heater supplies a small amount of a fluidizing gas to the heating zone, so that the fluidizing gas can be heated to the reaction temperature or above, without causing fluidization of the silicon particles in the heating zone. A pulsing device pulses the silicon particles back and forth, so that they can be periodically intermixed at the interface of the heating zone and the reaction zone, thereby maintaining the deposition reaction temperature. The application of pulsed physical impact to the bed of silicon particles using the pulsing device as proposed in U.S. Pat. No. 6,827,786 leads to forced intermixing of some of the silicon particles in the heating zone and the reaction zone. But, with this method, it is difficult to uniformly mix the particles while minimizing the temperature difference of the two zones in a large-sized reactor. Unlike other components commonly used in chemical processes, there is restriction in material selection of the components of the fluidized bed reactor. Particularly, the reactor tube in contact with which contacts the silicon particles should not be the source of impurity contamination in preparation of high-purity polysilicon. The reactor tube, the essential component of the fluidized bed reactor for polysilicon production, is in constant contact with hot fluidized silicon particles and is thus generally made of high-purity quartz or silicon to prevent impurity contamination. Because of irregular vibration and severe stress caused by the movement of silicon particles, the reactor tube is vulnerable to mechanical impact. Thus, the periodical application of physical impact to the bed of silicon particles using the pulsing device as disclosed in U.S. Pat. No. 6,827,786 may significantly impair the stability of the reactor tube and make the safe, sustained operation of the fluidized bed reactor difficult.
Accordingly, construction of the heating zone capable of solving the aforementioned problems and stably maintaining the reaction temperature in the reaction zone without affecting the mechanical stability of the fluidized bed reactor and an operation method thereof are prerequisites for large-scale production of granular polysilicon. Besides, supply of sufficient heat is necessary in order to significantly improve the productivity of the fluidized bed reactor by increasing the reaction pressure. It is important to construct and operate the fluidized bed reactor so that the heat supplied from the heating zone can be effectively utilized in the reaction zone, while maximizing the heat load in the heating zone.
Accordingly, an object of the present invention is to provide a method and an apparatus capable of improving the productivity of a fluidized bed reactor by stably maintaining the silicon deposition condition through sufficient supply of heat required for the preparation of granular polysilicon, without sacrificing the mechanical stability of the fluidized bed reactor.
To attain the objective, the present inventors completed the present invention based on the experimental finding that it is preferable that the construction and operation of a fluidized bed reactor for granular polysilicon satisfy the following conditions:
(1) the space that forms the bed of silicon particles within the reactor tube should be divided into a reaction zone and a heating zone by a reaction gas supplying means;
(2) the heat required for silicon deposition in the reaction zone is supplied by heating both the silicon particles in the heating zone and the fluidizing gas continuously passing through the zone using an internal heater installed at the inner space of the heating zone; and
(3) the silicon particles need to be intermixed between the reaction zone and the heating zone in a continuous, fluidized state, so that the heat supplied to the heating zone can be rapidly transferred to the reaction zone.